Barclay Crop Protection

Weed Control in Oilseed Rape

Many arable farms grow a predominance of cereal crops which are members of the grass family and the only non-cereal is often Oilseed Rape (which is a brassica). Apart from offering an excellent profit opportunity this crop represents a very good one to control weeds which may be becoming increasingly difficult to control elsewhere in the rotation. 

Plant Scientists have been extremely successful at developing selective herbicides that control non-desirable species from different families but it has proven much more difficult to control plants of the same family in any given crop. For example broadleaved weed control in cereals is relatively easy but far more difficult in broadleaved crops.

In addition oilseed rape seedlings are extremely vulnerable to competition from to early competition from grass weeds (inc vol. cereals).

Table 1: Relative importance of different weed species in winter oilseed rape:

*** = High.       **= Moderate   *= Low

1* if not herbicide tolerant.      *** if resistant to graminicides.

Source: HGCA/IACR Rothamstead

Hence effective control of grassweeds and cleavers is essential for yield optimisation.

Table 2: Weeds controlled by Propyzamide:

Grassweeds:                          Broadleaved Weeds:

Blackgrass.                              Chickweed.

Wild Oats                                Black Bindweed

Ryegrass                                 Redshank

Brome spp                               Fat Hen

Vol Cereals                              Knotgrass

AMG                                        Sowthistle

Rough stalked M. Grass             Cleavers (pre-em only).

In addition, many of the herbicides that do selectively control weeds of the same species have single modes of action (Eg: Sulphonyl-ureas) and as a result strains of weeds resistant to some of these materials have appeared fairly quickly and as a result some of them have become less effective. Some problems have already emerged with SU products and in order to retain it's efficacy for as long as possible it is important that farmers use both cultural and chemical techniques whenever and wherever possible.

When growing oilseed rape there is an excellent opportunity to use a herbicide with a different mode of action to that used anywhere else in the rotation such as propyzamide (the active ingredient in Propyz) which will not only be effective on the main target weeds like blackgrass, brome spp and wild oats but also control strains of these weeds which may be developing resistance to herbicides used in the cereal crops.

Examples of herbicides approved for use in Oilseed Rape are given below together with their modes of action.

Table 3:

Active Ingredient:         Susceptible Weeds:              HRAC Group            Crop Notes:

Propaquizafop              Grassweeds                           1                 Post-em only.

Clodinafop-propargyl   Grassweeds                            1                  Post-em only.

Tralkoxydim                 Grassweeds                          1                 Post-em only.

Metazachlor         Broadleaved + some grasses          K3               Pre/early post.

Clopyralid                    Some broadleaved                O                 Post-em only

Propyzamide        Grassweeds + Some broadleaved    K1               Post-em only. 

Propyzamide enters plants via the roots so ideally needs to be applied before the target species become too well established. This is not normally before November and but as it is broken down faster at higher soil temperatures early applications may not be advisable. The half life of propyzamide is roughly doubled when the soil temperature is 5C compared to 15C and this will help increase the persistence through to the spring.

Where grassweeds are well established before an application is made it may be advisable to tank mix with a low dose of an approved herbicide which has contact activity.

Summary:

1: Good crop establishment in the absence of weeds is essential.

2: Grassweeds are more important than broadleaved ones.

3: Try to avoid using the same types of chemistry as used elsewhere in the rotation (like Fops and Dims as in HRAC group 1).

4: Propyz is likely to be most effective if applied between 1^st November and 31^st December when soil temperatures have fallen.

5: Propyz offers a reliable cost effective opportunity to control a wide range of competitive weeds in all varieties of oilseed rape and will persist through the winter in almost all weather conditions.

Jonathon Harrington

Technical Manager, Barclay Chemicals